Investigation of the kinetics of the chain-radical process in the system iron carbonyl–Lewis base

An ESR spectroscopy investigation of the kinetics of accumulation and consumption of iron carbonyl radical anions, Fe₃(CO)₁₂⁻ and Fe₃(CO)₁₁⁻, arising in the reaction of Fe₃(CO)₁₂ with (Et₄N)SEt in THF at 19.5°C, was carried out using the stopped-flow technique. The solution of the inverse kinetic problem showed a satisfactory agreement between calculated and experimental data. This supports the principal idea that chain radical processes including preliminary complex formation followed by one-electron redox-initiation lie at the heart of the interaction of iron carbonyls with Lewis bases, and that the following transformations are due to electron and ligand changes in the coordination sphere of seventeen-electron coordinatively unsaturated species.     

Model of “living” active centers in kinetics of the solid-phase polymerization

A model of “living” active centers in the solid phase has been developed, which are the complexes of a traditional active center (radical or ion) with a carrier of excessive free volume in solids, the structural defect (vacancy, polyvacancy, dislocation, crack). This model for the first time successfully explained the most nontrivial kinetic peculiarities of the solid-phase polymerization.     

On the kinetics of pozzolanic reaction in the system kaolin–lime–water

The kinetics of the pozzolanic reaction of enriched kaolin from the “Senovo” deposit (Bulgaria) with lime is the object of this article. The kaolin contains kaolinite as a major clay mineral as well as admixtures of quartz and illite. The experimental data of pozzolanic activity at temperatures of 100 and 23 °C are obtained for different reaction times. The reaction degrees of kaolinite and lime at 100 °C are determined from the pozzolanic activity data using a powder X-ray diffraction analysis. The kinetic analysis is performed by joint presentation of theoretical and experimental data in dimensionless coordinates having in mind the influence of particle size distribution on the reaction rate. It is found by the kinetic analysis that the rate of entire reaction is limited by the rate of chemical reaction on the reaction surface up to degree of reaction near to 0.4. The rate of penetration of the chemical reaction into the kaolinite particles for this area—from the beginning to degree of reaction 0.4, is determined to be equal to 2.10⁻¹¹ m/s.     

The kinetics of the polymorphic transition of the α-form of 2,4,6,8,10,12-hexanitrohexaazaisowurtzitane

The α → γ polymorphic transition in hexanitrohexaazaisowurtzitane was studied by optical microscopy, calorimetry, IR spectroscopy, thermogravimetry, and X-ray diffraction. The thermal effect of the transition was determined. The kinetics of the process is complex because of the relation between structural rearrangement and transition with the removal of water stabilizing the structure of the α polymorph. Depending on the morphological characteristics of the initial sample, the polymorphic transition can follow a frontal-heterogeneous mechanism (single crystal → polycrystalline aggregate) or a quasi-homogeneous mechanism (single crystal → single crystal).     

The kinetics of chemical reactions in the products of dissociation of the CO2−H2 gas mixture in microwave discharge

The yields of hydrogen atoms, oxygen atoms and molecules, and hydroxyl radicals after a microwave discharge in the mixture of CO₂ and H₂ were measured by ESR spectroscopy in a flow-type system. A mathematical model of the kinetics of chemical reactions downstream the microwave discharge was devised. The concentrations of particles that cannot be detected under our experimental conditions were estimated. Experimental values of the concentration sensitivity for an RE-1306 ESR spectrometer are as follows: for a pressure of 1 Torr and optimized detection conditions, H^., 10¹¹ cm⁻³; O^., 3·10¹⁰ cm⁻³; OH^., 10¹⁰ cm⁻³; O₂, 3·10¹³ cm⁻³ (Ref. 7); for a pressure of 2 Torr, H^., 5·10¹² cm⁻³; O^., 2·10¹² cm⁻³; OH^., 2.5·10¹¹ cm⁻³; O₂, 7.5·10¹⁴ cm^{−3 8} Translated fromIzvestiya Akademii Nauk, Seriya Khimicheskaya, No. 4, pp. 665–669, April, 2000.     

On the kinetics of pozzolanic reaction in metakaolin–lime–water system

The kinetics of pozzolanic reaction metakaolin–lime is studied in the present work. Metakaolin is prepared by calcination of enriched kaolin (deposit “Senovo”, Bulgaria) at temperature of 830 ± 10 °C in a labscale muffle oven. The reaction is performed in intensively stirred water suspension at different temperatures in the range 20–100 °C. The kinetics is analyzed by comparing the experimental data with theoretical curves, derived according to appropriate kinetic and diffusion models taking into account the grain size distribution of metakaolin. The macroscopic mechanism and activation energy of the reaction are determined. It is found, that the activation energy decreases gradually from 71 to 45 kJ/mol[Ca(OH)₂] with the increase of the reaction degree from 0.2 up to 0.6, respectively, which is a characteristic for transition regime reactions.     

Does core size matter in the kinetics of ligand exchanges of monolayer-protected Au clusters?

This paper compares the kinetics of exchanges of phenylethanethiolate ligands (PhC2S-) of the monolayer-protected clusters (MPCs) Au(38)(SC2Ph)(24) and Au(140)(SC2Ph)(53) with p-substituted arylthiols (p-X-PhSH), where X = NO(2), Br, CH(3), OCH(3), and OH. First-order rate constants at 293 K for exchange of the first ca. 25% of the ligands on the molecule-like Au(38)(SC2Ph)(24) MPC, measured using (1)H NMR, vary linearly with the in-coming arythiol concentration; ligand exchange is an overall second-order reaction. Remarkably, the second-order rate constants for ligand exchange on Au(38)(SC2Ph)(24) are very close to those of corresponding exchange reactions on the larger nanoparticle Au(140)(SC2Ph)(53) MPCs. These are the first results that quantitatively show that the chemical reactivity of different sized nanocrystals is almost independent of size; presumably, this is because the locus of the initial ligand exchanges is a common kind of site, thought to be the nanocrystal vertexes. The rates of later stages of exchange (beyond ca. 25%) differ for Au(38) and Au(140) cores, the latter being much slower presumably due to its larger terrace-like surface atom content. The reverse exchange reaction was studied for Au(38)(p-X-arylthiolate)(24) MPCs (X = NO(2), Br, and CH(3)), where the in-coming ligand is now phenylethanethiol. Remarkably, the rate constants of both forward and reverse exchanges display identical substituent effects, which implies a concurrent bonding of both in-coming and leaving ligands to the Au core in the rate-determining step, as in an associative mechanism. X = NO(2) gives the fastest rates, and the ratio of forward and reverse rate constants gives an equilibrium constant of K(EQ,PE) = 4.0 that is independent of X.     

Oscillatory kinetics of the permanganate oxidation of oxalic acid?

The oxidation of oxalic acid by permanganate satisfies the mechanistic criteria of the oscillatory kinetics. However, the oscillatory changes in the absorbancy eventually found are due to formation of colloidal and coagulated MnO₂ and not to the chemical events.

---

. , , MnO₂, .

     

The kinetics,catalysis and inhibition of the Baeyer—Villiger reaction in the processes of liquid-phase oxidation of organic compounds

Literary data on kinetics, catalysis and inhibition of the oxidation reaction of carbonyl compounds with peroxy acids according to the Baeyer—Villiger reaction under aerobic liquidphase oxidation conditions have been considered and discussed. The main reaction channel involves a reversible formation of α-hydroxyperoxy ester and its rearrangement to an ester or a lactone. In the case of homolytic decomposition of α-hydroxyperoxy ester no esters are formed. At all steps the formation and transformation of α-hydroxyperoxy ester are catalyzed by carboxylic acids. The possibility of formation of the second intermediate, presumably dioxirane, is shown. Catalysts of the oxidation processes such as variable-valency metal salts influence the kinetics at all steps in the Baeyer—Villiger reaction. Inhibition of ester formation in the presence of cobalt and manganese salts is associated with catalysis of homolytic decomposition of peroxy acid and α-hydroxyperoxy ester.     

Effect of porosity on the kinetics of the two-route chlorine reaction: The chlorine evolution—ionization kinetics on iridium dioxide

The porosity effect on the kinetics of the chlorine reaction proceeding via two parallel paths is analyzed theoretically. An equation for the steady-state polarization curve is derived for these conditions. The chlorine evolution-ionization kinetics is studied by steady-state polarization measurements at a rotating disk electrode with an active iridium dioxide coating. A comparison of experimental and theoretical polarization curves shows the chlorine reaction to proceed via two parallel paths not only on DSA and RuO₂, but on IrO₂ as well     

Mechanism and kinetics of the production of hydroxymethyl hydroperoxide in ethene/ozone/water gas-phase system

The mechanism and kinetics of the production of hydroxymethyl hydroperoxide (HMHP) in ethene/ ozone/water gas-phase system were investigated at room temperature (298±2 K) and atmospheric pressure (1×105 Pa). The reactants were monitored in situ by long path FTIR spectroscopy. Peroxides were measured by an HPLC post-column fluorescence technique after sampling with a cold trap. The rate constants (k3) of reaction CH2O2 H2O→HMHP (R3) determined by fitting model calculations to ex-perimental data range from (1.6―6.0)×10?17 cm3·molecule?1·s?1. Moreover, a theoretical study of reac-tion (R3) was performed using density functional theory at QCISD(T)/6-311 (2d,2p)//B3LYP/6-311 G(2d, 2p) level of theory. Based on the calculation of the reaction potential energy surface and intrinsic reac-tion coordinates, the classic transitional state theory (TST) derived k3 (kTST), canonical variational tran-sition state theory (CVT) derived k3 (kCVT), and the corrected kCVT with small-curvature tunneling (kCVT/SCT) were calculated using Polyrate Version 8.02 program to be 2.47×10-17, 2.47×10-17 and 5.22×10-17 cm3·molecule-1·s-1, respectively, generally in agreement with those fitted by the model.     

The A53T mutation is key in defining the differences in the aggregation kinetics of human and mouse α-synuclein

Despite a 95% sequence similarity, the aggregation of human and mouse α-synuclein is remarkably different, as the human form is slower than the mouse form in forming fibrils but is associated with Parkinson's disease in both humans and transgenic mice. Here, the amino acid code underlying these differences is investigated by comparing the lag times, growth rates, and secondary structure propensities of a systematic series of eight human-mouse chimeras. Fluorescence analysis of these variants shows that the A53T substitution dominates the growth kinetics, while the lag phase is affected by a combination of the A53T and S87N substitutions. The secondary structure propensities derived from an NMR chemical shift analysis of the monomeric forms of the human-mouse variants enable us to establish a link between the changes in the conformational properties in the region of position 53 upon mutation and the corresponding changes in growth rates. These results suggest that the presence of an alanine residue at position 53 may be an evolutionary adaptation to minimize Parkinson's disease in humans and indicates that effective drug development efforts may be directed to target this N-terminal region of the sequence.     

Influence of protein hydrolysis on the growth kinetics of β-lg fibrils

Recently it was found that protein hydrolysis is an important step in the formation of β-lactoglobulin fibrils at pH 2 and elevated temperatures. The objective of the present study was to further investigate the influence of hydrolysis on the kinetics of fibril formation. Both the hydrolysis of β-lactoglobulin and the growth of the fibrils were followed as a function of time and temperature, using SDS polyacrylamide gel electrophoresis and a Thioflavin T fluorescence assay. As an essential extension to existing models, the quantification of the effect of the hydrolysis on the fibrillar growth was established by a simple polymerization model including a hydrolysis step.     

Crowding alters the folding kinetics of a β-hairpin by modulating the stability of intermediates

Crowded environments inside cells exert significant effects on protein structure, stability, and function, but their effects on (pre)folding dynamics and kinetics, especially at molecular levels, remain ill-understood. Here, we examine the latter for, as an initial candidate, a small de novo β-hairpin using extensive all-atom molecular dynamics simulations for crowder volume fractions φ up to 40%. We find that crowding does not introduce new folding intermediates or misfolded structures, although, as expected, it promotes compact structures and reduces the accessible conformational space. Furthermore, while hydrophobic-collapse-mediated folding is slightly enhanced, the turn-directed zipper mechanism (dominant in crowder-free situations) increases many-fold, becoming even more dominant. Interestingly, φ influences the stability of the folding intermediates (FI(1) and FI(2)) in an apparently counterintuitive manner, which can be understood only by considering specific intrachain interactions and intermediate (and hierarchical) structural transitions. For φ values <20%, native-turn formation is enhanced, and FI(1), characterized by a hairpin structure but slightly mismatched hydrophobic contacts, increases in frequency, thus enhancing eventual folding. However, higher φ values impede native-turn formation, and FI(2), which lacks native turns, re-emerges and increasingly acts as a kinetic trap. The change in the stability of these intermediates with φ strongly correlates with the hierarchical folding stages and their kinetics. The results show that crowding assists intermediate structural changes more by impeding backward transitions than by promoting forward transitions and that a delicate competition between reduction in configuration space and introduction of kinetic traps along the folding route is key to understanding folding kinetics under crowded conditions.     

On the predictions and limitations of the Becker-Döring model for reaction kinetics in micellar surfactant solutions

We investigate the breakdown of a system of micellar aggregates in a surfactant solution following an order-one dilution. We derive a mathematical model based on the Becker-D?ring system of equations, using realistic expressions for the reaction constants fit to results from Molecular Dynamics simulations. We exploit the largeness of typical aggregation numbers to derive a continuum model, substituting a large system of ordinary differential equations for a partial differential equation in two independent variables: time and aggregate size. Numerical solutions demonstrate that re-equilibration occurs in two distinct stages over well-separated timescales, in agreement with experiment and with previous theories. We conclude by exposing a limitation in the Becker-D?ring theory for re-equilibration of surfactant solutions.     

On the solvolysis kinetics of amidoesters derived from β-aminoalcohols

To better understand reactivity in such systems, fifteen amidoesters derived from β-aminoalcohols were solvolyzed at the ester group in mildly basic methanol-d₄. All trials showed pseudo-first-order kinetics by ¹H NMR. The rate constants are about 2 to 140-fold larger than those found with simple alkyl esters. The least bulky N-acyl groups generally sponsor the largest rate constants, and strongly so in two cases, but apparently not as a result of lesser steric crowding between the amide and ester groups. Rate constants are also greater for those amidoesters favoring an anti conformation at the amide linkage.     

Thermal analysis study of the influence of rapid solidification on the precipitation kinetics of theδ′ (Al3Li) Metastable phase in an Al-Mn-Li Alloy

The precipitation kinetics of the (Al₃Li) phase in two rapidly solidified and one conventionally cast samples of an Al-2.3Li-6.5Mn (in wt%) alloy are compared. Following high cooling rates, manganese is retained in solid solution in the aluminium matrix (Al) up to 6.0 wt%, far beyond the thermodynamic equilibrium value (0.36 wt% at 500°). Extended solid solution of manganese in aluminium induces strain gradients, similar to those produed by dislocations. The effect of such gradients, the size of which is proportional to the solute atomic fraction, is to enhance lithium precipitation, by lowering the activation energy, as observed, and also by affecting the rate parameter. Kinetic thermal analysis has been performed in a series of nonisothermal DTA experiments, operated in the heat flux DSC mode. The precipitation of the (Al₃Li) phase is evidenced by an exothermic peak whose characteristics were analyzed. The rate of transformation (precipitation) is assumed to obey the Johnson-Mehl-Avrami equation. The activation energy for the precipitation process, as well as the kinetic rate parameter have been evaluated for the rapidly solidified and the conventionally cast specimens. The activation energy for precipitation is lowered, from 105 kJ·mol^–1 for the conventionally cast material, down to 77 kJ·mol^–1 for a sample that exhibits manganese solid solubility extension of 6.00 wt%. The rate parameter for the precipitation reaction is reduced from about 1.40 for the slowly cast sample to 1.20 the rapidly solidified sample, as a result of additional stress-assisted precipitation.

---

Zusammenfassung Es wird die Präzipitationskinetik der (Al₃Li) Phase in zwei schnell erstarrten und einer gewöhnlich gegossenen Probe einer Al-2.3Li-6.5Mn (in Gew.%) Legierung miteinander verglichen. Bei hohen Abkühlgeschwindigkeiten wird Mangan in der Aluminiummatrix (Al) bis zu 6.0 Gew.% als feste Lösung zurückbehalten, weit über den thermodynamischen Gleichgewichtswert (0.36 Gew.% bei 500°C) hinaus. Derart ausgedehnte feste Lösungen von Mangan in Aluminium erzeugen Spannungsgradienten, ähnlich wie die durch Versetzung hervorgerufen. Die Wirkung solcher Gradienten, deren Größe sich proportional zum gelösten Atombruch verhält, steigert, wie beobachtet durch Minderung der Aktivierungsenergie und auch durch Beeinflussung des Geschwindigkeitsparameters die Präzipitation von Lithium. Die kinetische Thermoanalyse wurde mittels einer Reihe von nichtisothermen DTA-Versuchen durchgeführt. Die Präzipitation der (Al₃Li) Phase wird durch einen exothermen Peak bewiesen, dessen Charakteristik analysiert wurde. Die Geschwindigkeit der Umwandlung (Präzipitation) scheint der Johnson-Mehl-Avrami-Gleichung zu unterliegen. Aktivierungsenergie des Präzipitationsvorganges als auch der Geschwindigkeitsparameter wurden sowohl für schnell verfestigte als auch für gewöhnlich gegossenen Proben ermittelt. Die Aktivierungsenergie für die Präzipitation wird von 105 kJmol^–1 für das gewöhnlich gegossene Material auf einen Wert von 77 kJmol^–1 für eine Probe mit einem gelösten Mangangehalt von 6.00 Gew.% gesenkt. Der Geschwindigkeitsparameter für die Präzipitationsreaktion wird von etwa 1.40 für die langsam gegossene Probe auf 1.20 für die schnell verfestigten Proben gesenkt.

---

---

Dedicated to Professor Dr. H. J. Seifert on the occasion of his 60^th birthday     

The non-isothermal kinetics of mullite formation in boehmite–zircon mixtures

In this work, we studied the kinetics of mullite formation in different composites under non-isothermal conditions using DTA. Different composites based of mullite, alumina, zircon and zirconia were prepared by reaction sintering of boehmite (as alumina source) and zircon. Several mixtures were used while varying the percentage of the boehmite from 30 to 70 mass% with a step of 10. Five compositions marked as B30, B40, B50, B60 and B70 corresponding to boehmite–zircon ratios (mass%) of 30/70, 40/60, 50/50, 60/40 and 70/30 were fabricated and studied. The DTA conducted at heating rates of 10, 20 and 30 K min^?1 showed an endothermic peak in all composites at about 1,603 K associated with mullite formation. The activation energies measured from non-isothermal treatments for five compositions (30, 40, 50, 60 and 70 mass% of boehmite) were 1,029, 1,085, 1,262, 1,508 and 1,321 kJ mol^?1, respectively. The n values (Avrami parameter) for all compositions are larger than 2.5, the mullite crystallization of these composites is followed by three-dimensional growth.     

Role of the molecular weight and the composition on the hydrolysis kinetics of monolayers of poly(α-hydroxy acid)s

The hydrolysis kinetics of spread insoluble monolayers of poly(α-hydroxy acid)s with various molecular weights and lactic acid–glycolic acid molar ratios was followed by measuring simultaneously the decrease in the surface area at constant surface pressure and the evolution of the surface potential. The interfacial hydrolysis at alkaline pH leads to the progressive fragmentation of the polymer molecules and the appearance of charged insoluble products (detected by measuring the surface potential) and small soluble fragments (detected by measuring the decrease in the surface area). The data obtained by both approaches were interpreted in the framework of the random scission model. The rates of hydrolysis are larger for polymers with smaller initial polymerization numbers and increase with the decrease in the molar ratio of lactic acid units. Received: 7 December 1998 Accepted in revised form: 8 March 1999     

Monte Carlo simulation of phase separation kinetics in a concentrated polymer solution

Ｔｈｅｐｈａｓｅｂｅｈａｖｉｏｒｉｎｍｕｌｔｉｐｌｅｃｏｍｐｏｎｅｎｔｐｏｌｙｍｅｒｓｃｏｎｓｔｉｔｕｔｅｓａｌｏｎｇｓｔａｎｄｉｎｇａｃｔｉｖｅａｃａｄｅｍｉｃｓｕｂｊｅｃｔｂｏｔｈｉｎｐｏｌｙｍｅｒｓｃｉｅｎｃｅａｎｄｃｏｎｄｅｎｓｅｄｓｔａｔｅｐｈｙｓｉｃｓ.Ｉｔｉｓｅｓｐｅｃｉａｌｌｙｓｉｇｎｉｆｉｃａｎｔｉｎｇｕｉｄｉｎｇｔｈｅｆａｂｒｉｃａｔｉｏｎｏｆｐｏｌｙｍｅｒａｌｌｏｙｓ[1].Ｄｕｒｉｎｇｔｈｅｌａｓｔｄｅｃａｄｅｓｍｕｃｈａｔｔｅｎｔｉｏｎｈａｓｂｅｅｎｐａｉｄｔｏｔｈｅｃｏｍ…